1. Field of the Invention
The present invention relates generally to computed tomography (CT) and, more particularly, to a volumetric computed tomography (VCT) system.
2. Discussion of the Related Art
Computed tomography (CT) provides a transverse image of an object. Conventional fan beam CT uses a point x-ray source and a linear detector array. The detector array may have one or more detector rows. With a single rotation, one or more image slices can be reconstructed using computer algorithms.
In order to increase imaging speed, CT detector row number has been increased to many hundreds so that more image slices are acquired in each rotation. A wider detector array covers a larger field of view. Hence, a volumetric image can be reconstructed with a single gantry rotation. Such CT systems are often called volumetric CT (VCT) systems. VCT systems can use a two-dimensional detector, such as a flat panel imager. VCT systems that include a flat panel imager are commonly used in image guided radiotherapy and intervention as shown in U.S. Pat. No. 6,842,502, the entire contents of which are incorporated herein by reference. The patent describes an embodiment of a cone-beam computed tomography imaging system that includes a kilovoltage x-ray tube and a flat panel imager having an array of amorphous silicon detector. As a patient lies upon a treatment table, the x-ray tube and flat panel image rotate about the patient in unison so as to take a plurality of images as described previously.
FIG. 1 diagrammatically illustrates the geometry of a cone beam computed tomography (CBCT) system. CBCT systems usually include a point x-ray source 20 and a two-dimensional flat panel detector 22 mounted on a gantry. The source 20 and detector 22 rotates together about a central axis 24. The trajectory of source 20 is a full circle 26 or partial arc. X-ray beams generated by the source 20 are attenuated by the imaged subject. The attenuation measured by the detector 22 is used to reconstruct images of the object. With one full or partial rotation, a three-dimensional image of object 28 can be reconstructed using image reconstruction algorithms.
There are several disadvantages when using CBCT. For example, the flat panel detector may include a scintillation screen and a charge-coupled device photodiode array. The scintillation screen converts x-ray photons into visible light photons which are then detected by a photodiode array. The performance of such flat panel detectors, in the aspect of signal-to-noise ratio, detection efficiency and sampling speed, is inferior to discrete x-ray detectors that are used in a diagnostic helical computed tomography scanner. High noise level and low detection efficiency cause poor low contrast differentiation and noisier images. A further reduction in image quality may be caused by suboptimal performance of a flat panel imager. Approximate reconstruction artifacts exist when cone angle is large.
Another disadvantage of CBCT is that when x-ray beams pass through the object 28, x-ray photons are either absorbed or scattered. Since the x-ray detector is so wide, the scattered photons are likely to be detected by the two-dimensional detector 22. Scattered photons will add up on the images, and hence attenuation information cannot be accurately measured. Scatter causes artifacts in the images. CBCT images hence have low image quality than those from fan beam CT. Besides artifacts, scatter contamination also increases noise in the images. In order to compensate noise, stronger x-ray beams have to be used. Hence, x-ray exposure of CBCT imaging is also higher than fan beam CTs. Another problem with such a VCT system is the large cost of a flat panel detector.
Current techniques for scatter correction or rejection include calculating the scatter and then subtracting the scatter from the signal. However, the length of time the scatter calculation requires can be as long as hours or days using the Monte Carlo method. Furthermore, the noise from the scatter remains after the scatter profile has been subtracted from the signal, such that the signal-to-noise ratio decreases.
In another technique, the scatter is measured and then subtracted from the signal. This technique, however, subjects the patient to additional radiation exposure and prolonged scanning time and requires an additional scan to measure the scatter profile. Further, the noise from the scatter remains, which sacrifices the signal-to-noise ratio.
In yet another technique, a grid is positioned in front of the detector and behind the patient to block some scatter. However, the grid also partially blocks the primary x-ray beams, resulting in additional radiation exposure to the patient. Other techniques use an air gap by increasing the distance from the detector to the patient, which reduces the scatter that is collected by the detector. Because of mechanical limitations, however, the distance from the detector to the patient can be increased only a finite amount.
Other systems addressing the scatter problem are known. For example, a VCT system with a two-dimensional x-ray source array and a point or small detector is disclosed in U.S. Pat. No. 7,072,436, the entire contents of which are incorporated herein by reference. This approach is also called inverse geometry CT since the detector and source geometry is reversed. Compared to regular geometry VCT, the scatter component in inverse geometry VCT is very low due to the small detector. However, in practice it is difficult to make a large two-dimensional x-ray source array that can provide sufficient field of view. The two-dimensional x-ray source array is also cumbersome to be used in mobile CT scanners.
Another VCT geometry uses a linear array of x-ray sources, and a two-dimensional area detector as described in U.S. Pat. No. 7,072,436, the entire contents of which are incorporated herein by reference. Each x-ray source generates a fan beam perpendicular to the rotation axis. This system is able to reject scatter photons and perform exact image reconstruction. It also does not have beam divergence problem in the axial direction as cone beam CT.
Note that the use of multiple fan beams in computed tomography as described in U.S. Pat. No. 6,229,870 (“the '870 patent”), the entire contents of which are incorporated herein by reference, does not require expensive area detector. It also uses a linear array of x-ray sources, and the x-ray beam from each source is collimated to its own detector array. The fan beams are also perpendicular to the rotation axis. The gap between the detector arrays can be filled in by moving the imaging subject during gantry rotation. Multiple rotations are needed for generating an image and so the system described in the '870 patent is not a true VCT system.
Tetrahedron beam computed tomography (TBCT) is another VCT system that can reconstruct a three-dimensional volume in a single gantry rotation and is described in U.S. Pat. No. 7,760,849 and U.S. patent application Ser. No. 12/803,480, the entire contents of each of which is incorporated herein by reference. TBCT employs a linear detector array and linear source array which are orthogonal to each other. The linear detector array and linear source array form a tetrahedral volume instead of a cone volume of traditional CBCT. The beams from each individual source of the source array are collimated to fan beams so that scatter component is very low. TBCT does not require a very wide detector so costs are significantly reduced. In addition, a linear array of x-ray sources is relatively easier to make when compared with a two-dimensional source array. The approximate image reconstruction artifact due to cone angle can be eliminated or reduced by using iterative image reconstruction algorithms.
Similar to that of CBCT systems, the beams of TBCT are diverged (converged). In order to achieve certain field of view (FOV) at central axis, the linear source array and detector must be almost twice as long along their respective axes as the desired FOV along those axes. Moreover because of the divergence in axial (z) direction, the volume that received radiation is larger than the volume that can be reconstructed. A mobile CT scanner requires compact design so that it can be easily mounted on C-arm gantries.
Accordingly, it is an object of the present invention to reduce scatter generated in a volumetric computed tomography system.
Another object of the present invention is to provide for a compact volumetric computed tomography system.
Another object of the present invention is to reduce beam divergence in a transverse slice.
Another object of the present invention is to reduce the lengths of detector arrays.